Method of drying lumber



Patented Aug. 5, 1930 warren stares FATEbi-lf MORITZ L. MUELLER, OF SEATTLE, WASHINGTON, ASSIGNOR TO GENERAL DRY COMPANY, OF PORTLAND,

OREGON, A CORPORATION METHOD OF DRYING LUIKBER No Drawing. Original application filed. November 10, 1928, Serial No. 318,594.

tion filed July 3, 1929. Serial No. 375,798.

This invention relates to a method of dry ing lumber, and the present application is a division of my co-pending applicatlon, Serial No. 318,594, filed November 10th, 1928.

The parent application aforesaid relates to the use of centrifugal force, in the drying of lumber, and particularly to the use of centrifugal force in conjunction wlth other natural forces.

The present application is devotedto the treatment of lumber under the influence of a plurality'of forces applied seriatim. More particularly, the inventlon relates to the drying of lumber under the influence of centrifugal force, and the subsequent treatment of the lumber with heat or vacuum, or both, and either simultaneously or separately applied.

Under most of the lumber drying systems with which I am familiar, the lumber has been placed in drying kilns, and kept there long enough to remove substantially all of the free water, and as much of the combined water or moisture as possible. It is comparatively easy to get rid of the free water, but not so easy to get rid of the comb ned water. Under the present invention, it 1s proposed to very expeditiously get rid of the free water by centrifuging, and thereafter resort to the slower and more expensive removal of the combined water, by other means.

By virtue of this method, the time requlred for the complete drying of the lumber and the removal of both the free and combined water, is materially reduced. Further, the present method improves the quality of the lumber output, because it insures that allof the lumber that goes into the drylng kiln will be in a substantially uniform condition insofar as its degree of dryness is concerned. The initial centrifuging brings the whole mass of lumber to be dried to a substantially uniform degree of dryness before it is subjected to additional steps for the removal of the combined water. If these additional steps are to be carried out in an ordinary drying kiln, under the influence of heat, or a vacuum, or heat and vacuum, the lumber goes into such kiln with all of its constituent Divided and this applicaparts of a substantially uniform degree of dryness, as hereinbefore described.

As is set forth in my copending applicaleave the surface when sufficiently repelled,

or otherwise actuated.

When the moisture (liquid) content of the outslde portion of a piece of wood becomes lower than the inside portion, capillary attraction endeavors to effect an equalization of liquid content, with the result that liq uids travel from the wetter portions to the drier portions, through the cell walls of the wood. The cell walls permit this travel through the action of osmotic laws, which operate on many cellular structures include. mg wood.

The lumber driers now in general use are designed to set these two natural forces into action. Such driers may be divided into two classes, i. e.. atmospheric pressure driers, and vacuum driers.

V The action and result in the case of an at: mospheric pressure drier is as follows: The lumber is piled in such a way, that the various units (boards) are separated so as to ex: pose their surfaces to the surrounding atmosphere; the atmosphere is then heated to a temperature above that of the wood and the heat from the air is transmitted to the solid and liquid portions of the wood. This kinetic (heat) energy increases the vapor pressure of the wood liquids, which tends to force their gaseous product out of the wood. In addition, the heated atmosphere surrounding the wood dries the outer fibres of the same, causing the outer portions of the wood to the drier than the inner portion thereof, with the result that the liquids from the interiorof mosphere by saturation, which would stop capillary movement, the surrounding atmosphere is moved continuously so that the surface of the wood is constantly bathed in an atmosphere capable of maintaining the outer portions of the wood in a drier state than the inner portions thereof. During such movement of the atmosphere, it is suitably. conditioned with respect to temperature, hu

- midity, etc., to suit the particular conditions existing at the moment.

The operation and result in the case of vacuum type driers is as follows: The lumber is open piled in an airtight chamber, and the atmosphere is extracted from said chamber until conditions therein approach a vacuum. Under this reduction of atmospheric pressure, the force of vapor pressure, in its relation to surrounding pressure, is greater at the same temperature than under normal temperature pressure, and, in addition, the boiling point occurs at lower temperatures. As a result, the liquids in the wood vaporize and are forced (drawn) from the wood into the'lower pressures of the surrounding atmosphere. Since the exposed surfaces of the i wood may have their moisture removed. fas-.

ter than it comes from the inside portions of the material, capillary attraction usually assists in the transfer of the wood liquid. Driers of this type are usually operated at a temperature equal to, or above, the boiling point of the wood liquids, which results in the boiling off (converting moisture into vapor, transferring it through the wood as vapor, and removing it as vapor) the liquids in the Wood An investigation and review of the many lumber driers designed and in use throughout this country shows that the above described principles are exclusively used, and that differences in design lie in the methods of moving and conditioning the atmosphere within said driers in an endeavor to securemore positive and effective results, from the above described natural forces.

. In carrying out the invention, I revolveor swing the pieces of wood at a high speed about acentral point. The specific apparatus employed is not of the essence of the invention. It is common in the drying art to employ revolving wheels or cages within which material is placed and the water driven therefrom by the action of centrifugal force when said cages or wheels are rotated at, high speed. Devices of this character may be employed and the lumber may be loaded therein in any suitable Way. 1, preferably, load it with its of said lumber. Where the lumber is loaded with its width. parallel to the radii of the wheel or cage, it follows that the entrained free water need travel only a distance equal to the Width of the board. Liquids in wood are located in two parts of the wood cells,

part in the cavities. inside of the individual cells, called free water and part in physical combination with the cell walls proper, called combined water. Wood does not shrink with the removal of free water but does shrink with the removal of the combined water. Centrifugal force acts freely on the free water, but to a very limited extent on the combined water. Applying centrifugal force to a group of wood pieces having a variety of different initial moisture percentages to startwith, will remove the free water from each until it is gone and result in equalizing the moisture content of all at a point approximating that where all free water has been removed but no combined water. Consequently, there is no shrinkage.

The use of the centrifugal force in the manner described for initially removing the free water yields very important economical results upon a commercial scale.

Owing to extremes of moisture content in any one group of lumber which in commercial practice is gathered into a chamber for being dried together, the control of the osmotic forces becomes uncertain and difiicult; W'et groups of pieces enter stages during the drying process when osmotic forces should be stimulated or retarded to secure best results, at different times than drier groups, making it impossible for the process to be administered to produce the most efficient effect on all. This is directly due to the fact that osmotic forces edect both free water and combined water, making it possible for parts of a board or group of boards, to shrink while other parts of the same board or adjoining boards are still too wet toshrink. The result of this condition is the production of strains in the wood causing it to become unsuitable for its use, or to be sold at a lower price than normal, if allowed to materialize and causing an inefficient and slow application of the process, if guarded against.

After the lumber has been brought to a substantially uniform condition as to its dryness, by the removal of the free water, it is subjected to the action of heat or sub-atmospheric pressure, or to sub-atmospheric pressure in the presence of heat. The removal of the free water quickly and, consequently, cheaply, renders the whole process extremely,

economical, because, with the free water removed, the combined water can more readily respond to the action of the heat and/or, vacuum, than would be the case'if the free water were still present.

Further, the amount of combined water to be removed is so very small in comparison with the amount of free water to be removed that, with the time required for the removal of the free water eliminated, the remaining time required for the removal of the combined water is so short that the lumber can be very expeditiously brought to the desired dry state.

Further, it is the free water that is principally responsible for wide variations in conditions of lumber at the time that it is placed in a drying kiln, and it is the uneven drying, with the resultant checking and cracking that is chiefly responsible for the degraded condition found in some lumber products. The method of the present invention will go a long way toward correcting these conditions Having described my invention, what i claim is:

1. The hereindescribed method of drying lumber, which consists of subjecting the same to the action of centrifugal force until the free water is removed therefrom, and thereafter subjecting the same to the action of heat for the removal of the combined water.

2. The hereindescribed method of drying lumber, which consists of subjecting'a multiplicity of pieces of lumber having moisture contents of varying degrees to the action of centrifugal force for the removal of the free water and to bring all of said pieces of lumber to substantially the same condition of dryness, and thereafter removing the combined water by subjecting the lumber to the action of heat.

3. The hereindescribed method of drying lumber, which consists of subjecting a multiplicity of pieces of lumber having moisture contents of varying degrees to the action of centrifugal force for the removal of the free water and to bring all of said pieces of lumber to substantially the same condition of dryness, and thereafter removing the combined water by subjecting the lumber to the action of heat, in a partial vacuum.

4. The hereindescribed method of drying lumber, which consists of subjecting a multiplicity of pieces of lumber having moisture contents of varying degrees to the action of centrifugal force for the removal of the free water and to bring all of said pieces of lumber to substantially the same condition of dryness, and thereafter removing the combined water by subjecting the lumber to the action of a body of heated air.

5. The hereindescribed method of drying lumber, which consists of subjecting a multiplicity of pieces of lumber having moisture centrifulgal force for the removal of the free water and to bring all of said pieces of lumber to substantially the same condition of dryness, and thereafter removing the combined water by subjecting the lumber to the action of a body of heated air in a partial vacuum.

6. The hereindescribed method of drying lumber, which consists of subjecting a multiplicity of pieces of lumber having moisture contents of varying degrees to the action of centrifugal force for the removal of the free water and to bring all of said pieces of lumber to substantially the same condition of dryness, and thereafter removing the combined water by subjecting the lumber to the action of sub-atmospheric pressure.

7. The hereindescribed method of drying lumber, which consists of centrifuging the same in a direction across the lumber until the free water is removed and thereafter subjecting the lumber to heat, to remove the combined water.

8. The hereindescribed method of drying lumber, which consists of centrifuging the same in a direction transversely of the pieces of lumber until the free water is removed and the various pieces of lumber are brought to substantially the same condition of dryness, and thereafter subjecting the said pieces of lumber to the action of heat, to complete the drying by the removal of the combined water.

9. The hereindescribed method of drying lumber, which consists of centrifuging the same in a direction transversely of the pieces of lumber until the free water is removed and the various pieces of lumber are brought to substantially the same condition of dryness, and thereafter subjecting the said pieces of lumber to the action of a vacuum.

10. The hereindescribed method of drying lumber, which consists of centrifuging the same in a direction transversely of the pieces of lumber until the free water is removed and the various pieces of lumber are brought to substantially the same condition of dryness, and thereafter subjecting the said pieces of lumber to the action of heat in the presence of sub-normal pressure.

In testimony whereof I affix my signature.

MORITZ L. MUELLER.

contents of varying degrees to the action of v 

